A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
A position of a movable stage of the lithographic apparatus, such as a substrate table constructed to hold the substrate, is measured by position sensors. Encoder based position sensors are commonly used for position measurement. For example, a stationary grating is connected to a reference structure of the lithographic apparatus, while a plurality of encoder heads are connected to the substrate table. Each encoder head is arranged to, in cooperation with the stationary grating, measure a position quantity (in one or more dimensions), allowing to determine a position of the substrate table in respect of the stationary grating. In such encoder type position sensors, an encoder beam propagates along an optical path, which optical path propagates in part through a medium, such as air, synthetic air or another gas or gas mixture. A position measurement as obtained from the encoder type position sensor, may hence exhibit a dependency on a propagation characteristic (e.g. a refractive index) of such medium, making the encoder type position measurement dependent on factors such as temperature, humidity, etc. A purging of the lithographic apparatus by a purging gas has been proposed. Still however, a too large inaccuracy may be obtained due to factors such as pressure fluctuations, thermal fluctuations, humidity fluctuations, etc.